The need to safely install wheels on tractor/trailer combination vehicles has long been recognized, and numerous solutions have been made available. In 1962, a major producer of highway trailers introduced a new industry axle having a wheel retention system consisting of a single nut held in place by a cotter pin. Wheel installation using this single nut design was merely to tighten the wheel assembly in place and then back off the single nut to a position of loose clearance before placing the cotter pin in position to prevent rotational movement.
A potential weakness with all single nut designs is the inability to consistently eliminate all degrees of freedom between the threads of the spindle and those of the nut. Several tapered roller bearing manufacturers advised that a controlled amount of preload within the tapered roller bearings wheel assembly was not only acceptable, but it would be recommended if an accurate means could be achieved for that purpose. However, no known double nut configuration was capable of providing any measure of controlled preload on tapered roller bearings. In achieving the final jammed condition between the two nuts involved, it was always the case that the outer nut had to be tightened against the inner nut and during that process, additional pressure was brought to bear against the bearings.
The amount of additional preload pressure is related to the amount of clearance between the threads of the inner nut and the threads of the spindle. The tolerances of manufactured threads on axle nuts were so variable that inconsistent preload pressures would occur as a result of the inward movement of the inner nut through the space or lash between mating threads as the outer nut was tightened into the jammed condition. For this reason, the recommended practices by all manufacturers for installation of double nuts on tractor/trailer applications call for a back off of the inner nut prior to installing and tightening the outer nut.
My U.S. Pat. No. 5,795,037 describes a novel axle nut system adapted for use on the spindle ends of an axle which mounts a wheel assembly that includes tapered roller bearings. The system includes an adjustment nut, a washer and a retainer member. The adjustment nut is threaded into position having torque applied to it in a controlled manner to achieve specific preload bearing pressure against the outer bearings. Thereafter, the washer is placed into contact with the nut and is rotationally fixed with respect to the axle. Next, the threaded retainer member is rotated into contact with the washer.
With the threaded retainer member placed on the axle to prevent any outward movement of either the wheel assembly components or the adjustment nut and washer, the adjustment nut is rotated in the opposite direction by which it was installed until it jams forcibly against the washer and thus, the threaded retainer member. With this rotational movement of the adjustment nut, the original bearing pressure or preload exerted outward against the adjustment nut is transferred and bears against the washer and retainer member.
Since this patent issued, a number of different lock members have been tried in the interest of achieving the best tradeoff between reliability and convenience of installation. These various methods included bend-over tabbed washers, and the use of a setscrew to associate the retainer nut with the intermediate washer that engages the keyway of the axle spindle. Both of these systems were used over the course of the last several years, but both were also criticized by those who perform the installation procedure.
The use of the bend-over tabbed washer was somewhat compromised because it was necessary for the washer to be thin enough to facilitate bending yet strong enough to prevent shearing of the tab within the keyway at the time the inner adjustment nut was jammed outward against the outer nut or lock member. The finally selected thickness provides dependable resistance against shearing, but it was too thick to facilitate ease of bending of the locking tab over the outer nut. Also, the use of the bend-over tab was difficult because space constrictions within the hub cavity complicated assembly and disassembly operations.